In oil and gas production systems, unstable flow in flow lines might cause serious and troublesome operational problems for the downstream receiving production facilities. Typical flow lines are pipelines, wells, or risers. Common forms of flow variations are slug flow in multiphase pipelines and casing heading in gas lifted oil wells. In both cases the liquid flows intermittently along the pipe in a concentrated mass, called a slug. The unstable behaviour of slug flow and casing heading has a negative impact on the operation of oil and gas production system, such as offshore facilities. Severe slugging can even cause platform trips and plant shutdown. More frequently, the large and rapid flow variations cause unwanted flaring and limit the operating capacity in the separation and compression units. This reduction is due to the need for larger operating margins for both separation (to meet the product specifications) and compression (to ensure safe operation with minimum flaring). Backing off from the plant's optimal operating point results in reduced throughput.
Three approaches are conventionally practiced to manage the instabilities in wells, pipelines, or risers:                Choking the flow        Increasing the gas lift rate        Providing overcapacity to accommodate the gas and liquid slugs        
Recently, a new alternative method using automatic feedback control was disclosed in the international application WO 02/46577. This method uses measurements of pressure, flow, or temperature as input to an automatic feedback controller for the purpose of stabilizing the flow by continuously manipulating the flow line outlet choke/valve. The measurements are taken upstream of the point where the main part of the slug is formed or is about to occur. Studies using other measurements than the inlet pressure for stabilization are disclosed in E. Storkaas and S. Skogestad: “Cascade Control of unstable systems with application to stabilization of slug flow”, presented at IFAC-symposium Adchem '2003. The authors use linear feedback controllers, which continuously manipulate the outlet valve opening in order to stabilize the flow line.
However, one significant challenge of operating an inlet pressure feedback controller such as the one in WO 02/46577, is to select the inlet pressure set point given to the controller. Some rules of thumb are given in WO 02/46577 without disclosing any specific solution. The set points given to the feedback controller in WO 02/46577 are also assumed to be manually selected/changed. To realize the importance of the inlet pressure set point, it should first be noted that flow rates into the flow line generally will increase if the flow line inlet pressure decreases. This means that in order to maximize the production from the flow line, its inlet pressure should be kept stable and as low as possible by the feedback controller. However, one cannot use an arbitrary low set point for the inlet pressure. Firstly, it might be impossible for the controller to stabilize the flow line at a too low set point. Secondly, the controllability, that is, the ability to control the inlet pressure using the flow line outlet valve, might become poor. This is due to the fact that by lowering the set point, the valve will typically operate at a valve opening which is, in average, larger. This again implies that the pressure drop across the valve might become very small.
The pressure drop, dP, across the valve gives a measure of the influence changes in the valve opening will have on the fluid movements in the flow line.
In addition, experience has shown that sudden drops in the liquid outflow from the flow line and the associated dP across the outlet valve may occur also after the flow line has been stabilized. The result is poor controllability of the flow line, meaning the outlet valve opening will have little or no effect on the outlet liquid flow. This means that if there is an automatic feedback control law manipulating the outlet valve opening, this will lose control over the flow line and flow instabilities will occur if the flow line is unstable without using feedback control.
As an example, let P1 denote the flow line inlet pressure, P2 the upstream valve pressure, P3 the downstream valve pressure and let the pressure difference across the valve be denoted by dP=P2−P3. The valve is assumed to be located at the outlet of the flow line. The inlet flow to the flow line will normally increase if P1 decreases. If dP decreases at the same time, indicative of the liquid outflow rate from the flow line is being reduced (assuming a constant valve opening), a mass imbalance in the flow line results. Hence, a liquid blockage in the flow line is probable to occur. In addition, if for example a standard linear PID (Proportional+Integral+Derivative) controller is used for controlling the flow line inlet pressure, the controller might order the valve to reduce its opening (depending on the tuning and choice of inlet pressure set point). The result of this will be an even lower outlet flow. Also, a sudden reduction of the liquid flow out of the pipeline/well may not be sufficiently observable in the inlet pressure before it is too late, that is, before the liquid plug has been established in the flow line. Hence, control laws using only measurements at the flow line inlet for feedback control will probably fail in preventing the drop in the outflow. Therefore, maintaining controllability of the flow line, that is, preventing the liquid outflow from the flow line from approaching zero even for a stabilized flow line, is a significant challenge.
FIG. 5 shows real-site data of a stabilized pipeline. The inlet pressure P1 exhibits relatively small variations (up to time=5 hours). However, at time=3.25 hours, a sudden decrease in the pressure drop across the valve dP occurs. At the same time, P1 is also decreasing. This eventually results in the building-up of a liquid slug and an unstable pipeline flow. This can be observed in the inlet pressure from time=5 hours.
It is however not necessary that a decrease in the inlet pressure takes place in order for a drop in dP to be problematic. This is illustrated by the real-site data shown in FIG. 6. At time=6 hours, a sudden drop in dP results in an unstable pipeline although P1 does not decrease whilst the sudden drop in dP takes place.
For an overview of prior art control methods for stabilization of flow lines, reference is made to the international application WO 02/46577 and its cited references. However, none of the methods in these references, including the method described in WO 02/46577, address the specific problem of preventing that a sudden drop in the liquid outlet flow results in poor flow line controllability, possibly liquid blockage, and eventually an unstable flow line.